I recently received a brand new Raspberry Pi, model B. Getting started with it was a bit non-straightforward, so here is what I could understand.

Flashing the Debian image on a SD card

download the OS image from the raspberry pi website, or even better with bittorrent

plug a SD card on a SDCard reader

diskutil list to list all available disks. Let's say your sdcard is disk2

umount your sdcard, with diskutil unmountDisk /dev/disk2

dd if=debian-pi-os.img of=/dev/rdisk2. This operation may take a while (like an hour or so).

whenever you what, your can ask dd to print the current status of the copy with killall -INFO dd or type Ctrl-T in the terminal
Starting the ssh server on boot

The OS that can be downloaded from the raspberry pi website is built with the ssh server turned off by default for security reasons. You can easily enable the ssh server with the following instructions :

plug the SDCard on your SDCard reader. The SDCard has several partitions, the first one is VFAT so readable / writable from the Macos Finder